To a person skilled in the art, the term water glass refers to vitreous, water-soluble alkali metal silicates (i.e. alkali metal salts of silicic acids) solidified from the melt, or viscose aqueous solutions thereof. In water glass, from 1 to 5 mol of SiO2 are typically present per 1 mol of alkali metal oxide (Alk2O—where Alk represents the alkali metals lithium, sodium and potassium), which is why sodium and potassium water glasses are usually also characterized by the ratio % SiO2 to % alkali metal oxide. Water glasses are thus chemically alkali metal silicates having a molar ratio (MR) of SiO2 to Alk2O in the range from 1 to 5, where this molar ratio is the molar ratio of the building blocks SiO2 and Alk2O constituting the alkali metal silicate. They contain oligomeric silicate anions with alkali metal cations as counterions.
Water glasses in the pure state are colorless glasses which form colloidal clear, alkaline solutions with water at elevated temperature and pressure. Water glass is usually prepared by melting silica sand with sodium carbonate or potassium carbonate at from 1400 to 1500° C., with the silica driving off the CO2. The solidified melt is either sold commercially in the milled state or is converted into an aqueous solution of the desired concentration. Aqueous solutions have the advantage of better applicability in a range of different fields of use.
Liquid water glass solutions, i.e. aqueous solutions of alkali metal silicates, are produced industrially from solid alkali metal silicate lumbar glass by pressure dissolution of the latter in water in autoclaves. As an alternative, sand can also be directly hydrothermally dissolved in concentrated alkali metal hydroxide solution, but only solutions having a low MR can be obtained.
As is known to those skilled in the art, a melt of alkali metal silicate is firstly formed in the preparation of water glass in a melting furnace, and this melt solidifies on cooling to room temperature to form glass pieces (known as lumbar glass). These glass pieces are industrially dissolved in water in pressure autoclaves, usually at elevated temperature, for instance at about 120° C. (known as the dissolution process). Industrial water glass solutions usually have a solids content of from about 30 to 50% by weight.
Industrial water glasses are generally not prepared using very pure quartz as SiO2 raw material source because of the lack of availability in large quantities but instead are produced using sand, and technical-grade water glass solutions therefore usually contain traces of particular metals. Commercial water glass solutions therefore contain, regardless of the production route, metallic impurities due to the raw material, with these impurities being present in total amounts below 0.3% and in the case of aluminum below 0.2%, usually significantly below, in clear, storage-stable aqueous solutions; as usual, these percentages are percent by weight of the respective metal oxide based on the total aqueous solution of the alkali metal silicate. The reason why the content of “metallic impurities” in water glasses is only so low is that only “pure” alkali metal silicates are water-soluble and only very small proportions of two- or three-valent metal cations form water-insoluble compounds. Since proportions of insoluble material would interfere in virtually all applications because of their tendency to separate out, they are separated off in industry by sedimentation, centrifugation or filtration in order to obtain clear, transparent storage-stable water glass solutions.
It is also generally known that water glass solutions react very sensitively to addition of even small amounts of aluminum-containing solutions, usually by immediate gelling and delayed precipitation of solids. There is therefore an in-principle difficulty when the content of aluminum in water glass solutions is to be set to a value above about 0.2-0.3% (percent by weight of Al2O3 based on the total aqueous solution of the alkali metal silicate). This has been confirmed in in-house tests by the applicant. Here, the applicant has found that an attempt to carry out a direct synthesis of clear aqueous alkali metal silicate solutions having a relatively high aluminum content from the inexpensive raw materials sand, NaOH and an aluminum source by the hydrothermal production route is also unsuccessful.
Mixtures of water glass solutions with aluminate solutions are used industrially to prepare aluminosilicates as amorphous molecular sieves or crystalline zeolites. This illustrates, from another perspective, the abovementioned difficulty of producing clear water glass solutions having an elevated aluminum content. Studies by the applicant have confirmed that combining water glass solutions with aluminate solutions leads to precipitation or crystallization, with the liquid phase being depleted in aluminum so that an alkali metal silicate solution having virtually no Al content remains as mother liquor after the precipitation is complete.
Mitra, Transaction Ind.Ceram.Soc, Vol XXXI 1972, p.31ff describes Alk/Si/Al variations within a wide range, dilutions and temperatures in the range 0-95° C., but with the result that all mixtures gel or become turbid after not more than 24 hours.
In American Mineralogist, V64, 1979, p.172ff, Ueda indicates that clear solutions of aluminates in water glass solutions can also exist but with the restriction that this is possible only at Al2O3/SiO2 ratios in the range from 1:70 to 1:320, which for a conventional water glass solution having an alkali metal silicate solids content of 40% by weight mathematically means that Al2O3 can be present in a maximum amount of 0.4% by weight. This prior art thus confirms to a person skilled in the art the known fact that it is extremely difficult to prepare clear aqueous solutions of alkali metal silicates (water glasses) which have an appreciable aluminum content and are stable on storage in the long term and do not gel and indicates that it could be possible to prepare such solutions having a maximum aluminum content of at most 0.4% by weight.